A semiconductor sensor includes a silicon substrate having a hollow portion, an insulation film covering the hollow portion and disposed on the front side of the substrate, and a heater generating heat by being energized and disposed on the insulation film. The heater is made of semiconductor layer.
The above sensor is disclosed in JP-A-H07-58346. The sensor is suitably used for a gas flow sensor for detecting gas flow amount. The gas flow sensor includes a hollow portion, a nitride film, a doped silicon film and a heater. The hollow portion is formed in a silicon substrate in such a manner that a back side of the substrate is etched. The nitride film is formed on a front side of the substrate, and covers the hollow portion. The doped silicon film is deposited on the nitride film. The heater is formed such that the doped silicon film is patterned to be a predetermined shape. The heater is energized so that the heater generates heat. When the gas flows through the heater, the gas conducts the heater away from the heater so that the electric resistance of the heater is changed. Based on the resistance change of the heater, the gas flow amount is detected.
Here, in a pressure sensor, a concavity is formed in a silicon substrate, and a bottom of the concavity provides a diaphragm. A sensing element is formed on the diaphragm. Thus, the diaphragm is not removed from the substrate. Accordingly, a part of the substrate, on which the sensing element is formed, has large mechanical strength.
In the above gas flow sensor, the heater as a sensing element is heated to generate the heat. Thus, it is necessary to heat the sensing element with small current for a short time. Accordingly, it is necessary to reduce heat capacity of a part of the device, on which the heater is formed. Further, it is necessary to increase insulation property of the part of the device. Thus, the part of the device provided by a film has small thickness. In this case, an insulation film made of a nitride film having a large thickness covers the hollow portion of the substrate, and the heater is formed on the nitride film.
However, stress may be applied from an opening of the hollow portion toward the bottom of the hollow portion. Specifically, when the gas flow sensor is arranged in an intake manifold of an internal combustion engine, the stress is generated. In this case, the stress may be caused by pulsation when backfire is caused. The stress is concentrated at a boundary between the sidewall and the bottom of the hollow portion.
Accordingly, the insulation film on the hollow portion may be removed from the silicon substrate, so that the strength of the part of the device, on which the heater is formed, is reduced.
Thus, it is required for the gas flow sensor to improve mechanical strength so that the insulation film at the boundary between the sidewall and the bottom of the hollow portion is not removed from the silicon substrate.